There are cases where it is necessary to administer a drug, such as an analgesic, a sedative, an antipyretic, or a nutritional supplement, into a patient's body continuously for a predetermined period of time. As a continuous drug solution infusion device used for this purpose, there has heretofore been known a device including a single tube having a drug solution inlet part and a drug solution outlet part attached respectively to both ends thereof. The device further includes a balloon made of an elastic material, e.g. a rubber material, and secured at both ends thereof to the outer periphery of the tube. The respective insides of the tube and the balloon are communicated with each other through a communicating hole provided in the tube. With this continuous drug solution infusion device, a drug solution is stored into the balloon from the drug solution inlet part through the communicating hole, and the stored drug solution is forced to flow out through the communicating hole by the contracting force of the expanded balloon and continuously infused into the patient's body from the drug solution outlet part. The drug solution outlet part is provided with a control passage to suppress a high-rate flow of drug solution caused by the pressure of the balloon and to administer the drug solution into the patient's body at a predetermined low flow rate over a long period of time. With the conventional continuous drug solution infusion device, however, the flow rate is high at the beginning of the infusion and decreases with the passage of time. Therefore, it is difficult to infuse the drug solution into the patient's body substantially at a predetermined flow rate from the beginning of the infusion. The reason for this is as follows. Because the balloon is secured to a single tube, it can expand only in the radial direction of the tube and cannot expand in the axial direction of the tube. Accordingly, the pressure of the balloon changes to a considerable extent from the time of expansion to the time of contraction.
Meanwhile, there is a continuous drug solution infusion device in which one end of a balloon is secured to a single tube to allow the balloon to expand and contract in both the radial and axial directions of the tube. With this device, however, a protection cover secured to the tube needs to be increased in the axial dimension in order to allow the expansion of the balloon in the axial direction, which requires a relatively large accommodating space at the time of storage and transportation. In addition, there is an increase in the cost of disposing of the device as a medical waste.